Motor control having means to prevent release of contactor during motor acceleration



Dec. 20, 1966 M. J. BOYAJIAN MOTOR CONTROL HAVING MEANS TO PREVENT RELEASE OF CONTACTOR DURING MOTOR ACCELERATION Filed March 27, 1964 FEWZM United States Patent MOTOR CONTROL HAVING MEANS TO PREVENT RELEASE OF CONTACTOR DURING MOTOR ACCELERATION Myron .I. Boyajian, Chicago, IlL, assignor to Allis Chalmers Manufacturing Company, Milwaukee, Wis.

Filed Mar. 27, 1964, Ser. No. 355,160 7 Claims. (Cl. 318-484) This invention relates generally to motor controls; more specifically to a motor starting circuit.

The motor control of this invention is intended to operate in a situation that will be illustrated by an electric fork lift truck operation. Fork lift trucks of the type that will be described here have hydraulic systems which raise and lower the load supporting carriage and tilt the mast on which the carriage is reciprocably mounted. The hydraulic system has an electric motor driven pump and manual controls that turn on the motor to start the pump and position a valve to select the direction of moving either the mast or carriage. These manual controls are located conveniently near the truck operator and occasionally the truck operator may accidently brush against the operating levers and thereby energize the pump motor only momentarily. Such momentary operation is undesirable because the motor contactor is opened while the motor current is much higher than it is at running speed. One object of this invention is to prevent momentary operation of an electric motor. When the motor is energized it begins accelerating its mechanical load and at first draws a heavy current, for example about 100 amperes. In the normal operating situation the motor accelerates to its running speed where the motor current is much lower, and after the mast or carriage has been adjusted the truck operator manually deenergizes the motor. In momentary operation however, the motor contactor is opened during the initial accelerating period when the current is much higher than during normal running. In interrupting the heavy starting current, the motor produces an arc between its contacts that increases the wear on its contacts and from some contactor designs makes the truck unsafe for operating in an explosive atmosphere.

One object of this invention is to provide a new and improved motor control that prevents the motor circuit from being interrupted during the short interval in which the motor draws the heavy starting current. Another object of this invention is to provide an electric motor operated system in which the motor turnoff is delayed enough to prevent the motor contactor from interrupting its circuit during the accelerating period, but in which the motor operated load device can be operated for any selected interval With-out regard to the delay.

The motor control circuit of this invention includes a slow release contactor that is given a time delay characteristic that is matched to the motor accelerating characteristic to operate the motor long enough after its manual control has been released to assure that the motor has accelerated to its normal running speed.

Stated from another point of view, an object of this invention is to provide a new and improved motor control that responds to motor current to prevent interrupting the motor circuit when the current is above a preselected value.

In the drawing, FIG. 1 is a top view of a fork lift truck using the motor control of this invention;

FIG. 2 is a schematic of the motor control system of this invention; and

FIG. 3 is a graph showing an illustrative time-current characteristic of a motor and a corresponding time delay characteristic that is given to the motor control of this invention.

Patented Dec. 20, 1966 Introduction FIG. 1 shows a fork lift truck having a chassis 10 supported by wheels 11 and carrying a mast 14 that supports a carriage 9 to which forks 13 are secured. A pair of hydraulic cylinders 15 are interconnected between the truck chassis 10 and the mast 14 and can be energized to tilt the mast forwards and backwards in a conventional manner. A conventional chain mechanism that is not shown in the drawing connects a hydraulic cylinder 17 to the carriage 9 to raise and lower the load carrying forks 13 on the mast. The hydraulic system of cylinders 15 and 17 comprises a double spool valve mechanism 19 and hydraulic lines 20 that connect a pump 21 to energize cylinders 15 and 17 according to the position of valve mechanism 19. Valve mechanism 19 is controlled manually by two operating handles 24, 25 connected to valve mechanism 19 through linkages 26, 27 each of which includes a cam 28 (FIG. 2) only one of which is shown. The two operating handles 24, 25 are positioned close to the operators seat 30. The hydraulic system also includes a reservoir 31 for pump 21.

Pump 21 is driven by an electric motor 32 that is controlled by a contactor 33 to be energized from the truck battery 34 (FIG. 2). Contactor 33 has a pair of load terminals 35, 36 connected to control the circuit of motor 32 and a coil 38 with two terminals 39, 40 that operates a magnetic armature and movable contact assembly 41 to open and close the circuit between terminals 35, 36.

The Control The control for contactor 33 includes a control switch 45 and a cam follower 48 that is operated by either cam 28 to close the circuit between control switch terminals 46, 47 whenever either operating handle 24, 25 is moved in either direction. The control also includes a time delay release circuit 49 comprising a capacitor 50 and a resistor 51 connected in circuit with coil 38 to cause contactor 33 to release a predetermined time after control switch 45 has been opened.

The components of the time delay circuit are given the appropriate values to delay the release of motor contactor 33. These values of the release circuit will be explained as the operation of the circuit is described.

Operation Suppose that operating handles 24, 25 are in the neutral positions, contactor 33 and control switch 45 are open, and motor 32 is stopped. When either of the handles 24, 25 is moved in either direction the associated cam 28 operates contuol switch 45 and connects coil 38 and time delay release circuit 49 to be energized by battery 34. The current in coil 38 rises rapidly to the pullin value (FIG. 3) to close the circuit between terminals 35 and 36 and energize motor 32. When motor 32 is energized, its current rises rapidly to a high value that may be about amperes. Then, as the motor accelerates, its current is decreased by the counter E.M.F. that the motor develops. After the time required to accelerate the mechanical load on the motor, the motor current reaches a value that depends on the motor load. In the fork lift truck system that illustrates the invention, the hydraulic system and pump 21 establish rather uniform starting and operating conditions for motor 32 and the illustrative motor current curve of FIG. 3 for a particular system does not change much from one operation to the next; the combination of valve 19, cam 28, and control switch may be constructed so that handles-24 or 25 first close switch 45, while valve 19 is in a bypass position, and then operate a valve 19; this construction isolates motor 32 from the mechanical load on carriage 9 and thereby tends to maintain a uniform starting load whenever the handles 24, 25 are accidently brushed against. The curve of FIG. 3 can be considered to represent the slowest accelerating condition that the control is subjected to. Consequently, it is practicable to select a time (labeled delay in the drawing) that is long enough for the motor to accelerate to about its normal running speed. The time delay release circuit is designed to provide a corresponding delay. When operating handles 24, 25 are both returned to their neutral position to move cams 2.8 to open control switch 45, the circuit energizing coil 38 comprises only coil 38, and capacitor 50 and resistor 51. Since capacitor 50 has been charged with its terminals having the same polarity as coil 38 during energization of the coil, the capacitor begins to act as a voltage source energizing coil 38. Because energy is stored in the magnetic field of coil 33, the coil also begins to act as a voltage source. The current in coil 38 and capacitor 50 decays according to a well known formula (see Skilling, Electrical Engineering Circuits, John Wiley & Sons, Inc., pages 472 to 477).

The time delay circuit is designed to allow the current to decay to the relay drop out value (FIG. 3) at the end of the time delay. The appropriate values of resistance and capacitance for the inductance of the coil are. selected according to the formula to provide this delay. Resistor 51 is given enough resistance to suitably limit the current in the time delay circuit when control switch 45 is closed to prevent excessive wear on the contacts of control switch 45. Resistor 51 is also given enough resistance to prevent the circuit of coil 38 and capacitor 50 from oscillating with enough amplitude that the coil would pick up on the half cycle after it had opened. Resistor 51 is given enough conductance to charge capacitor 50 within the shortest time that control switch 45 might be held closed.

The control provides a longer opening delay than closing delay because the impedance of coil 38 is much higher than impedance of the battery 34 and switch 45 and because coil 38 provides some energy for the time delay.

For some controls the energy stored in the field of coil 38 may by itself provide a suitable time delay, and means such as a diode might be provided in place of the resistor 51 and capacitor 50 to provide a circuit for maintaining current flow in coil 38.

Those skilled in the art will recognize the variety of applications for the motor accelerating control of this invention and variations of the specifically disclosed control within the scope of the claims.

Having now particularly described and ascertained the nature of my said invention and the manner in which it is to be performed, I declare that what I claim is:

1. A control for a motor energized from a DC. source, the motor having a uniform starting load whereby the motor requires substantially a constant predetermined time interval to accelerate to about its running speed, comprising,

a motor switch having load terminals which are open when said motor is idle and are adapted when closed to connect said motor to said source and complete an energizing circuit to said motor and having input terminals and responsive to an input at said input terminals to close the circuit between said load terminals,

2. control switch for connecting said motor switch input terminals to the DC. source to close said motor switch, and being adapted when released to disconnect said motor switch input terminals from said D.C. source, and

electrical time delay means connected in shunt with said input terminals of said motor switch and being operative upon opening of said control switch for preventing said motor switch from releasing for a time interval after opening of said control switch approximately equal to said predetermined time interval for accelerating.

Z. A control according to claim 1 which said motor switch comprises a contactor having fixed and movable contacts and a magnetic coil connected across said input terminals for actuating said contacts.

3. A control according to claim 2 in which said time delay means comprises a circuit connected across said coil for maintaining current flow in said coil after said control switch is opened.

4. A control according to claim 3 in which said circuit comprises a capacitor having a capacitance value selected to cause the current of said coil to decay to its dropout value at the end of said predetermined time.

5. A control according to claim 4 in which a current limiting device is connected across said coil in series with said capacitor, said current limiting device having suflicient impedance in the direction of the capacitor charging current to limit the current to a value within the current rating of said control switch and having sufficiently low impedance in the direction of discharging the capacitor to maintain the current above the dropout value during said predetermined time.

6. A control in accordance with claim 1 and including manually actuated handle means for operating said control switch and for subsequently operating means to mechanically load said motor.

7. A control in accordance with claim 4 wherein the time-current characterictic of said time delay means along which the current through said coil decays to said dropout value is approximately parallel to the time-current characteristic of said motor while said motor is accelerating to its running speed.

References Cited by the Examiner UNITED STATES PATENTS 1,917,418 7/1933 Almquist et al. 317-142 2,779,906 1/1957 A-driansen 318- 184 X 2,899,619 8/1959 Wiedemann 3l8-484 X 3,004,202 10/1961 Sirnson 318447 X 3,134,939 5/1964 Eddy 3l8-485 X ORIS L. RADER, Primary Examiner.

G. A. FRIEDBERG, Assistant Examiner. 

1. A CONTROL FOR A MOTOR ENERGIZED FROM A D.C. SOURCE, THE MOTOR HAVING A UNIFORM STARTING LOAD WHEREBY THE MOTOR REQUIRES SUBSTANTIALLY A CONSTANT PREDETERMINED TIME INTERVAL TO ACCELERATE TO ABOUT ITS RUNNING SPEED, COMPRISING, A MOTOR SWITCH HAVING LOAD TERMINALS WHICH ARE OPEN WHEN SAID MOTOR IS IDLE AND ARE ADAPTED WHEN CLOSED TO CONNECT SAID MOTOR TO SAID SOURCE AND COMPLETE AN ENERGIZING CIRCUIT TO SAID MOTOR AND HAVING INPUT TERMINALS AND RESPONSIVE TO AN INPUT AT SAID INPUT TERMINALS TO CLOSE THE CIRCUIT BETWEEN SAID LOAD TERMINALS, A CONTROL SWITCH FOR CONNECTING SAID MOTOR SWITCH INPUT TERMINALS TO THE D.C. SOURCE TO CLOSE SAID MOTOR SWITCH, AND BEING ADAPTED WHEN RELEASED TO DISCONNECT SAID MOTOR SWITCH INPUT TERMINALS FROM SAID D.C. SOURCE, AND ELECTRICAL TIME DELAY MEANS CONNECTED IN SHUNT WITH SAID INPUT TERMINALS OF SAID MOTOR SWITCH AND BEING OPERATIVE UPON OPENING OF SAID CONTROL SWITCH FOR PREVENTING SAID MOTOR SWITCH FROM RELEASING FOR A TIME INTERVAL AFTER OPENING OF SAID CONTROL SWITCH APPROXIMATELY EQUAL TO SAID PREDETERMINED TIME INTERVAL FOR ACCELERATING. 